Magnetoresistive random access memory (MRAM) is one of the several types of random access memory in development that will likely serve as alternative to the main stream flash memory design. It will maintain a nonvolatile status while retaining the attributes of high density, high speed, and low power. The core technology difference between MRAM and other types of nonvolatile RAM is the way in which it defines and stores digital bits. Thin magnetic films are stacked in a structure called a magnetic tunnel junction (MTJ) and it is the variation in electrical resistance of current that pass through the two alternating state of magnetization of this MTJ structure that defines the digital bits (“0” and “1”) for MRAM. The memory bit element can be programmed by a magnetic field created from pulse current carrying conductors above and below the junction structure. In a newer design of MRAM, a spin transfer switching technique can be used to manipulate the memory element as well. This new design will allow better packing and shrinkage of individual MTJ devices on the wafer, effectively increasing the overall density of the MRAM memory elements.
Recent developments in the field of magnetic sensors have led to the incorporation of MTJ devices to aid ultra-sensitive applications such as bio-sensors and current sensors. The ability to use an array of MTJ devices, to amplify the change in an internal or external magnetic field disturbance, will greatly increase the sensitivity of the magnetic sensor. In these applications, the ability to place the magnetic sensing device (generally a MTJ structure) as close as the source of the field disturbance as possible is critical to the success of the device.
After the deposition of the various required layers, the individual MTJ structures are formed by photolithography and reactive ion etching (RIE). A dielectric layer such as silicon oxide is then deposited to isolate the individual MTJ structures from one another. This is followed by planarization of the surface to facilitate subsequent photolithography using Chemical Mechanical Polishing (CMP). This is needed to flatten the surface and fully expose the top conductive cap layer of the MTJ for proper electrical contact to the next metal interconnect layer.
CMP is widely used in semiconductor fabrication to planarize a non-planar top surface during the processing of semiconductor wafers. The process uses a slurry of a corrosive chemical along with an abrasive, in conjunction with a polishing pad, to effect the planarization. The pad and wafer are pressed together by a dynamic polishing head and held in place by a plastic retaining ring. This removes excess top layer material and any irregular topography, making the wafer surface planar. One particular type of CMP involves the use of a highly selective slurry (HSS), which has the ability to remove different materials at significantly different removal rates. The HSS process has a tendency whereby the removal rate drops rapidly with decreasing pressure between the wafer and the CMP pad, caused by the diminishing topography on the surface of the wafer. CMP has been successfully adapted for use in shallow trench isolation (STI) as part of conventional semiconductor processing.
The magnetic tunnel junction structure serves as the critical core for storing a single bit of the MRAM. Prior to depositing the MTJ films, the area in which the MTJ device will be built should be made to be flat or slightly convex relative to the surrounding dummy (inter-device) areas. This is needed because, after most of the silicon oxide above the MTJ structure has been removed from most of the wafer, the removal rate of silicon oxide decreases dramatically due to the reduction in pressure between the wafer and the CMP pad surface. In order to ensure complete removal of the silicon dioxide on top of the MTJ all across the wafer, the device block needs to protrude slightly above the nearby dummy areas in order to generate an adequate silicon oxide removal rate. This can be accomplished by adjusting the oxidizing agent in the final slurry used in the metal CMP process of the previous layer. A decrease in oxidizing agent concentration will cause the rate of metal removal to be lower than that of the surrounding dielectric. This will allow the metal interconnect layer, or in the design of spin transfer MRAM, the metal via layer right below the MTJ structure, to be either flat or slightly protruded at the device block location.
A HSS is usually used as part of the shallow trench isolation (STI) process in the fabrication of conventional semiconductor devices. In that context, the STI process uses silicon nitride as a capping layer for the device structure, the latter acting as a top stop layer for the active silicon underneath. This is schematically illustrated in FIG. 1. Seen there are a series of semiconductor devices 12 on a common substrate 11 and embedded in dielectric layer 13. Each device 12 has a silicon nitride cap 14. As CMP proceeds, the reduced removal rate over the hard silicon nitride causes shallow trenches 16 to gradually be formed with a minimum amount of dishing. CMP is stopped when feedback from the polishing system indicates that the polish rate has reached a predetermined low value.
Previous processes for manufacturing MRAMs have typically used a via hole structure for forming the electrical connection between the MTJ and the metal interconnections located in the level above.
A routine search of the prior art was performed with the following references of interest being found:
U.S. Pat. No. 7,241,632 and U.S. Patent Application 2006/0234445 (both to Yang of Headway) teach CMP to expose the MTJ stacks using HSS. SiN spacers are formed on the sidewalls of the stacikes to prevent shorting after CMP. U.S. Pat. No. 7,245,522 (Aoki) teaches CMP to expose MTJ elements for electrical connection without a via hole process. No CMP details are provided. U.S. Pat. No. 6,956,270 (Fukuzumi) discloses CMP to expose MTJ stacks for connection to subsequently formed bit lines. No CMP details are provided. U.S. Patent Application 2005/0153561 (Jin et al) shows a highly selective slurry with selectivity of 100:1.